Allow posix_fadvise in capability mode

posix_fadvise operates only on a provided fd. Noted by
Mathieu <sigsys@gmail.com> in review D34761.

No new CAP_ rights are added for posix_fadvise(), as 'advice' in
general only influences when I/O happens; the fd must have existing
CAP_ rights for actual data access.

Reviewed by: markj
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34903